Because the United States has become increasingly dependent on foreign sources of natural gas and petroleum, recent focus has shifted to the abundant domestic sources of coal to replace a portion of the foreign sources of natural gas and petroleum. Switching from foreign energy sources to domestic coal will lead to price stability and increased national security.
Coal remains an important source of fuel for electricity generation and a feedstock for coke making and chemical production. Coal is preferred to natural gas and petroleum because of its secure domestic supply and relatively low, stable price. However, using coal requires more extensive processing than natural gas and petroleum to mitigate dust and gas emissions and other environmental concerns. Many investigations have been undertaken to mitigate dust, nitrous oxide, and sulfur dioxide emissions from coal-fired power plants and other sources that use coal. Additionally, other pollutants such as mercury have recently come under scrutiny by governments and health organizations. As a result, future regulations may force coal users to significantly reduce mercury and other heavy metal emissions at great cost.
Coal gasification offers favorable environmental benefits and high energy conversion efficiencies relative to traditional combustion methods used by pulverized, coal-fired power plants. Gasification occurs when coal is placed in a vessel under high temperature and pressure and mixed with steam and oxygen. The organic materials contained in coal are converted into carbon monoxide, carbon dioxide, hydrogen, and other compounds. The combustible components carbon monoxide and hydrogen are typically separated from non-combustible water vapor, carbon dioxide and other gases. The mixture of combustibles, often referred to as synthesis gas or “syngas,” provides a feedstock for combustion turbines and gas-to-liquid processes such as the Fischer-Tropsch process. The Fischer-Tropsch process converts syngas into valuable salable organic chemicals such as distillate fuels, naphtha, and wax. Combustion turbines are typically arranged in a combined-cycle configuration to produce more electricity from a given amount of syngas.
The efficiency of gasification, and subsequent gas-to-liquid reactions in a Fischer-Tropsch process, largely depends upon the quality and specific energy content of the coal feedstock. High-quality feedstocks provide better conversion efficiencies than low quality materials resulting in reduced carbon dioxide emissions from high-quality feedstocks.
Countries with few natural gas or petroleum reserves, such as Germany and South Africa, rely on coal and the Fischer-Tropsch process to produce liquid fuels. Low quality coal reserves, especially those found in abundance in the western United States in abandoned coal waste piles in Appalachia, and in many other areas in the world, could be economically useful if a practical method were available to enhance the quality of the coal. Upgrading low-rank coals and lignite by reducing the amount of sodium present would greatly improve the utilization value of the fuel. A significant amount of sodium in the coal is associated with basic compounds that dissolve in acids and other chemicals. Unfortunately, the high cost of these acids and other reagents make the upgrading process economically unattractive to both coal producers and users. The cost of the feedstock (coal, natural gas or petroleum) constitutes the majority of the overall cost of gasification and conversion. The cost of coals increases with the quality and therefore an inexpensive process that would upgrade coal, particularly poor-quality coal, prior to gasification is needed to enhance the overall economics of gasification and conversion. In particular, the process would be beneficial if it reduced ash minerals such as those containing sodium, potassium, calcium and magnesium, elements that often create difficulty with gasification and combustion, to levels that make the use of low rank coals economically feasible.